Preferential Activation of Primary CH Bonds in the Reactions of Small Alkanes with the Diatomic MgO^+. Cation

Abstract

The C-H bond activation of small alkanes by the gaseous MgO(+*) cation is probed by mass spectrometric means. In addition to H-atom abstraction from methane, the MgO(+*) cation reacts with ethane, propane, n- and iso-butane through several pathways, which can all be assigned to the occurrence of initial C-H bond activations. Specifically, the formal C-C bond cleavages observed are assigned to C-H bond activation as the first step, followed by cleavage of a beta-C-C bond concomitant with release of the corresponding alkyl radical. Kinetic modeling of the observed product distributions reveals a high preference of MgO(+*) for the attack of primary C-H bonds. This feature represents a notable distinction of the main-group metal oxide MgO(+*) from various transition-metal oxide cations, which show a clear preference for the attack of secondary C-H bonds. The results of complementary theoretical calculations indicate that the C-H bond activation of larger alkanes by the MgO(+*) cation is subject to pronounced kinetic control.

Models in Catalysis

“…does not react with methane [24], and neither do larger cations. It needs an alkane with a more reactive C-H bond, e.g.…”

“…For the neutral V 4 O 10 with a tetrahedral arrangement of four vanadyl groups, O = V(O-) 3 [23], experiments see Ref. [24] Models in Catalysis 113…”

Chemie mit Methan: Studieren geht über Probieren!

“…Wie schon erwähnt, ist die direkte Wasserstoffabstraktion durch relativ kleine KIEs charakterisiert; die indirekten metallvermittelten homolytischen Spaltungen von C-H-Bindungen weisen hingegen grçßere KIEs auf, die typischerweise bei Werten > 2.0 liegen. Wie von Schrçder et al eingehend diskutiert, [50] steht die Grçße der KIEs für einen bestimmten Reaktionstypus ferner in einem Zusammenhang mit der Gesamtgeschwindigkeit der Wasserstoffabstraktion. Die Zunahme der intermolekularen KIEs von 1.3 für MgO + C, über 2.1 und 2.4 für CaO [53] Die direkte Bildung von Methanol und Formaldehyd aus Methan unter Normalbedingungen [2] birgt mçglicherweise das grçßte Potential für eine grundlegende Verbesserung der Technologie der Methan-Konvertierung.…”

CH Activation on Aluminum–Vanadium Bimetallic Oxide Cluster Anions

“…Gas-phase clusters have well-defined structures and may have definite active sites that parallel the active sites on the surface of real catalytic systems. Selective activation of C À H bonds by various gas-phase clusters has been extensively studied, and many valuable results have been reported concerning the activations of methane, [4, ethane, [13,34,[37][38][39] propane, [13,34,[37][38][40][41][42] and butane. [13,37,40] However, most of these gas-phase studies were carried out on cationic clusters, [3, 4, 10-11, 14-29, 38-40, 43] and only a few recent studies have focused on anionic clusters.…”